Optic Modes in Amorphous ${\mathrm{As}}_2$${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$

New reflectance spectra are obtained for amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$. The spectra contain features not reported in earlier studies. Analysis of the spectra in terms of damped Lorentzian oscillators, combined with new Raman data, gives further support to the validity of a molecular model for the optic-mode vibrational frequencies. Using mode masses derived from the model, values of the macroscopic effective charge $e_T^{*}$ are obtained. For both materials, $e_T^{*}$ values are comparable to those of other covalently bound solids; e.g., trigonal Se and trigonal Te. Contrary to early suggestions, it is demonstrated that the large values of $e_T^{*}$ in these materials are not a manifestation of ionic bonding but rather result from dynamical charge transfer. In both ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ there are sufficient differences between the infrared (ir) and Raman spectra so that they cannot be interpreted simply in terms of a density of states by assuming a complete breakdown of selection rules due to disorder. On the contrary, selection rules resulting from the short-range order or the molecular nature of these solids are of considerable importance in determining the ir or Raman activity of the optic modes.